1. Field of the Invention
The present invention relates to a wireless communication system useful in a wireless LAN for performing communication by space division multiplexing, and a wireless station included in this system.
2. Description of the Related Art
A wireless LAN system based on IEEE 802.11 (ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 edition) using CSMA (Carrier Sense Multiple Access) is known. This wireless LAN system can consist of plural stations and at least one access point. A station performs carrier sensing before transmitting a packet to the access point. This carrier sensing includes physical carrier sensing and virtual carrier sensing. Physical carrier sensing checks whether a wireless medium is busy or idle, from a received signal level. Virtual carrier sensing checks whether a wireless medium is busy or idle, from reservation information contained in a received signal.
The station performs carrier sensing and postpones packet transmission, starts connecting to the access point, or transmits the packet. That is, when a reception level of a signal is larger than a certain threshold value or when a packet containing channel reservation information (NAV (ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 edition)) is received from another station or access point, the station postpones packet transmission and when the wireless communication medium becomes idle after the elapse of a random transmission backoff time, the station starts connecting to the access point. When the station is already connected to the access point, the station transmits a packet in which the address of the access point is designated without waiting random time.
SDMA (Space Division Multiple Access) is known as one multiplexing method in a wireless communication system. SDMA is implemented at an access point by using an adaptive array antenna. The adaptive array antenna forms plural antenna beams which reduce interference between them. This improves the communication quality and realizes simultaneous communication between an access point and plural stations.
It is presumably possible to achieve similar advantages by applying this SDMA to a wireless LAN system based on CSMA.
If, however, SDMA is simply applied to a CSMA wireless LAN system, the following problem arises.
Generally, a station does not have a directional antenna such as an adaptive array antenna. Therefore, while a certain station is transmitting a packet to an access point, another station determines by the carrier sense function described above that the wireless medium is busy, and postpones its packet transmission. Accordingly, even if an access point of a wireless communication system using CSMA has an adaptive array antenna, space division multiple communication in which plural stations perform simultaneous communication through the same channel cannot be efficiently performed.